(1) Field of the Invention
The present invention generally relates to an exhaust emission control apparatus for an internal combustion engine, and more particularly to an exhaust emission control apparatus for an internal combustion engine in which electric power is applied to a heating element of a catalytic converter mounted in an exhaust passage of the engine, and a catalyst of the catalytic converter is heated by the heating element to accelerate a rate of catalytic conversion of exhaust gases.
(2) Description of the Related Art
An exhaust emission control apparatus including a catalytic converter is provided in an exhaust passage of an internal combustion engine to carry out catalytic conversion of exhaust gases from the engine. This catalytic conversion of exhaust gases implies catalytic activity by a catalyst in the catalytic converter for accelerating oxidization of carbon monoxide (CO) and hydrocarbon (HC) in the exhaust gases and/or accelerating reduction of oxygen nitride (NOx) in the exhaust gases. However, the catalyst in the catalytic converter is not active and the efficiency of the catalytic conversion is extremely low when the temperature of the catalyst is still low, that is, it has not yet reached a catalyst activation level. Once the engine starts operating, the temperature of the catalyst is increased due to the heat of exhaust gases passing into the exhaust passage. But, when the engine undergoes a cold start and has still not warmed up, the heat of exhaust gases is low and the temperature of the catalyst does not rise quickly to the above mentioned catalyst activation level. The engine in such a condition exhibits a poor level of fuel combustion efficiency, and it is difficult for efficient catalytic conversion of exhaust gases to be performed by a catalyst with a low temperature, that is below the catalyst activation level.
In the prior art, there is a disclosure of a catalytic converter for an internal combustion engine. For example, Japanese Laid-Open Utility Model Application No. 49-124412 discloses a catalytic converter including a catalyst and a heater provided within the catalytic converter for heating the catalyst to accelerate the rate of the catalytic conversion of exhaust gases. Electric power is applied to the heater of the converter for heating the catalyst in the catalytic converter. A sensor is provided for detecting whether or not the temperature of exhaust gases is higher than a prescribed standard temperature, so that it is possible to detect whether or not the temperature of the catalyst has reached a catalyst activation level responsive to an output signal of the sensor. When it is detected that the temperature of exhaust gases has reached the prescribed standard temperature, the application of electric power to the heater of the catalytic converter is stopped responsive to the output signal of the sensor. However, in this conventional catalytic converter, the sensor mounted in an exhaust passage downstream of the catalytic converter is subjected to an extremely high temperature due to the heat of exhaust gases passing into the exhaust passage. Thus, there is a problem in that the sensor in the conventional catalytic converter is very likely to experience major malfunctions, such as a short circuit or an open circuit. If a short circuit should occur in the sensor, a temperature of exhaust gases detected by the sensor is not increased to a correct temperature, which will cause occurrence of malfunctions in the apparatus. Malfunctions in the apparatus can cause such problems as overheating of the catalyst, damage to the catalytic converter or damage to the battery. Also, if an open circuit should occur in the sensor, the sensor may erroneously send a signal indicating a temperature much higher than the standard temperature. In such a case, electric power is not applied to the heater for heating the catalyst and the catalyst does not become active, thus causing a reduced rate of catalytic conversion of exhaust gases.
The conventional catalytic converter also has a problem in that a time period during which electric power is applied to the heater of the catalytic converter cannot be adjusted, and it is difficult to attain effective catalytic conversion of exhaust gases especially when a temperature of the catalyst is very low or very high before application of electric power to the heater is started.